1. Field of the Invention (Technical Field)
The present invention relates to the thermal management aspect of a lasing thin disk which is nominally 200 microns (10−6 meters), 1-2 cm in diameter and crystalline/ceramic material like Yb:YAG, tungstates, sesquioxides and others. The emphasis of this novel thermal management of the thin disk laser is an attempt to achieve much more uniform temperature profile across the lasing thin disk and efficient heat conduction for the lasing thin disk to greatly improve its beam quality (BQ) thus enabling much more widespread use.
2. Description of Related Art
Note that the following discussion may refer to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
High power solid-state lasers (SSL) have become the emphasis for industrial and commercial applications due to their efficiency, compactness and small supporting infrastructure relative to high power electrical or chemical gas laser system. Scaling SSL to high powers (>kW's and approaching 100 kW's) has required specific attention to thermal management. Over the last 15 years, thin disk laser has been developed and matured in Germany into reliable laser systems operating at kW's of average powers and used extensively for welding, cutting and material processing. In spite of this success, there are several issues needed to be improved to advance TDL technology to next level, including (1) thermal management; (2) power limitations by amplified spontaneous emission (ASE); (3) thin-disk gain material fabrication for reliable operation and (4) improved homogenization of the pumping laser diode radiation to acquire nearly perfect “flattop” intensity profiles. This invention focuses on the thermal management aspect of the lasing thin disk.
The thin disk laser (TDL) pioneered by Giesen has demonstrated high powers (>4-5 kW) and “wall plug” efficiencies better than 20% as discussed by A. Giesen, et al., “Scalable concept for diode-pumped high-power solid-state lasers”, Appl. Phys., Vol. B 58, 363 (1994) and J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws”, IEEE J. Sel. Top. Quant. Electr., Vol. 13 (3), p. 598 (2007). FIG. 1 illustrates the concept consisting of a lasing thin disk (TD), a hemispherical resonator having back side of thin disk serving as a flat mirror, a cooled heat sink for the TD and the laser diode pump radiation coming from a parabolic multi-pass reflector assembly not shown as disclosed in U.S. Pat. Nos. 4,921,041; 6,438,152; 6,577,666; 6,963,592; 7,003,011; and 7,200,160. Although only a part of the pump beam is absorbed by the thin gain element, the pump efficiency can be optimized by re-imaging the pump radiation several times with an optical system of a parabolic mirror and redirecting mirrors. Up to 32 passes of the pump beam can be used with more than 90-98% of the pump power absorbed in the disk. The output laser power at kW levels is multi-transverse mode with M2 values from 5-20 and single, lowest order mode cw operation occurs only for a few 100's of watts.
Even with these impressive results, it is important to achieve much higher single mode laser operation with good beam quality (BQ) from thin disk lasers. Improved high power BQ operation of TDL would enable many more applications. The diminished BQ performance at high powers, however, is the most significant shortcoming of this promising laser technology and is caused by the thermal behavior of the lasing thin disk. As the TD is heated by the optical pumping radiation, it experiences non-uniform temperature profiles which creates a dynamic lens plus significant thermally-induced stress. These stresses produce both time and spatial varying birefringence in the disk that causes laser polarization changes and non-spatial phase changes across the thin disk. This polarization change and the phase changes, respectively, reduce the laser energy extraction from the thin disk and further degrade the TDL beam quality. All of these thermal problems are attributed to insufficient thermal management of the thin disk. To date, nearly all high power TDL use jet impingement cooling which produces a spatially non-uniform cooling of the thin disk. In this patent application, the use of a special variation of heat pipes produces significantly improved isothermal cooling of the thin disk that will lead to greatly improved beam quality plus good energy extraction from the thin disk laser. This invention should greatly expand the application of the TDL.
The main difference between TDL and conventional rod or slab lasers is the geometry of gain medium. For the TDL, the thickness of the laser crystal (or ceramic) is quite small, 100-200 μm and the diameter is typically 1-2 cm. The large surface-to-volume ratio of the thin disk like fiber lasers makes possible the efficient removal of heat from the TDt. As heat is removed via diffusion through the back side of the TD, the temperature distribution in the radial direction could be made quite uniform provided the central area of the disk is pumped by a near flat-top intensity profile and the diffusive cooling is very efficient. Today's operating TDL, however, do not have these ideal pumping and cooling conditions. The present invention deals with the thermal management of the thin disk in an attempt to achieve much more uniform and efficient conduction cooling for the TD's. The most commonly employed approaches for the thin disk lasers use jet impingement cooling of two configurations shown in FIG. 2. These types are the “cold plate” (or “cold finger”) thin disk and the “capped” thin disk. In the former, the thin disk is bonded to a “cold plate” (usually 1 mm thick CuW for Yb:YAG lasing thin disk) which is cooled on the back side via water jet impingement. [D. J. Womac, “Correlating Equations for Impingement Cooling of Small Heat Sources with Single Circular Liquid Jets”, Transactions of ASME, Vol. 115, p. 106 (1993)] In the “capped” thin disk, an approximate 1 mm thick, undoped crystal or polycrystalline ceramic (like YAG) is bonded to a thin disk like Yb:YAG. The Yb:YAG is then cooled directly to maximize the diffusion of heat through the 200 micron thick lasing material (Yb:YAG is this case) as disclosed in U.S. Pat. Nos. 6,600,763 and 7,200,216. The 1 mm thickness material provides mechanical strength for the nominal 200 micron laser gain material. The most widely used gain medium for TDL is Yb:YAG as discussed by C. Stewen, et al, “A 1-kW cw thin disc laser”, IEEE J. Sel. Top. Quant. Electr., Vol. 6 (4), p. 650 (2000); K. Contag et al., “Theoretical modeling and experimental investigations of the diode-pumped thin disk Yb:YAG laser”, IEEE J. Quant. Electr., Vol. 29 (8), p. 697 (1999); D. Kouznetsov, et al., “Surface loss limit of the power scaling of a thin-disk laser”, J. of Opt. Soc. Amer., Vol. B-23 (6), p. 1074 (2006). Lately many other system has been demonstrated like Yb:KGW as discussed by G. Paunescu, et al., “100-fs diode-pumped Yb:KGW mode-locked laser”, App. Phys. B, Vol. 79, p. 555 (2004); J. E. Hellstrom, et al., “Efficient Yb:KGW lasers end-pumped by high-power diode bars”, Appl. Phys. B, Vol. 83, p. 235 (2006), Yb:KYW as discussed by K. Seger, et al., “Tunable Yb:KYW laser using a transversely chirped volume Bragg grating”, Optics Express, Vol. 17 (4), p. 2341 (2009)] and Yb:Lu2O3 as discussed by R. Peters, et al., “Broadly tunable high-power Yb:Lu2O3 thin disk laser with 80% slope efficiency”, Opt. Express, Vol. 15 (11), p. 7075 (2007).
Unfortunately, as related above, non-optimum conduction cooling creates a non-uniform uniform temperature across the thin disk surface and in the TD itself as shown in FIG. 3 for the two specific geometric configurations shown in FIG. 2. Such non-isothermal temperature profile conditions cause a thermal lensing effect plus thermal-stress induced birefringence which results in the laser power decreasing due to depolarization loss. A thin disk of a high power TDL has very high power loading in the disk (up to 100's of kW/cm3 absorbed pump power density). To date, approximately 100 W of output power in a diffraction-limited continuous-wave (CW) beam has been obtained with a single disk. Multimode TDL with 6.5 kW output power from a single disk has been demonstrated. [A. Lobad, et al., “Characterization of a Multikilowatt Yb:YAG Ceramic Thin-Disk Laser”, J. of Directed Energy, March, 2011] The scaling laws show that the power limit for CW operation can be theoretically projected to be beyond 40 kW for one single disk. [J. Speiser, “Fifteen years of work on thin-disk lasers: results and scaling laws”, IEEE J. Sel. Top. Quant. Electr., Vol. 13 (3), p. 598 (2007)] These non-uniform temperature profiles in a lasing crystal disk result in thermal distortion of the output beam and degradation of laser operation due to thermal stress induced birefringence and deformation in the thin disk. FIGS. 3 and 4 show the non-uniform temperature profile in the radial direction and the temperatures increases in the supporting structure for the TD. In FIG. 4, note the significant difference between the “cold-finger” and “capped” configurations of the Yb:YAG thin disk across the radial direction. These temperature variations for the entire thin disk holding assembly also cause significant deformation for the entire structure. FIG. 5 illustrates this effect for both TD configurations. FIG. 6 shows the axial expansion in a 1 cm pumped cross section versus the radius. Any laser beam propagating through this effective, thermally induced lens will create significant detrimental effects on the TDL's ultimate BQ and resulting performance.
Relation of OPD to Beam Quality: Thin Disk Temperature Control Needs
Besides the thermal lensing effects caused by non-uniform expansion of the thickness, the change of refractivity index (n) varies with the temperature of lasing thin disk. For a thin disk of thickness d having an integrated refractivity Δn along its axial direction (FIG. 7) and operating at a wavelength λ, the laser optical phase difference as a function of the radius. [K. M. Swift, R. D. Rathge and L. A. Schlie, “Dispersion of Gases in Atomic Iodine Lasers at 1.315 μm”, Appl. Optics, Vol. 27, p. 4377 (1988)].Δφ(r)=2Ω*Δn*d/λ  (10)For a good beam quality, the condition Δφ≦λ/10 needs to exist. At room temperature for Yb:YAG, the thermal-optical coefficient dn/dT=9×10−6/° K. [R. Wynne, et al., “Thermal Coefficients of the Expansion and Refractive Index in YAG”, Appl. Opt., Vol. 38, p. 3182 (1999)] To achieve this condition that Δφ≦λ/10, the maximum temperature variation across the thin disk must be <20° K. A very good beam quality should have Δφ≦λ/20 and an excellent BQ should be Δφ≦λ/50 which corresponds, respectively, to temperature variations across the thin disk to be less than 10° K and 2° K. To meet this requirement, a very good thermal cooling method is required that can provide a much more uniform temperature distribution across and through the thin disk. The results of FIGS. 4-5 and 9 shown below clearly illustrate that jet impingement cooling will not improve the BQ for the TDL. Besides the degradation in BQ due to these non-uniform temperatures, the temporal variations of the temperature profiles will cause many different modes to have transient behavior and operation.
Heat generated during the operation of High-Average-Output (HAP) solid state lasers causes thermal distortion in the laser amplifier that can be detrimental if not properly controlled. The mode structure and quality of the output beam depends on the amount of volumetric heat deposited into the amplifier, the shape and geometrical configuration of entire laser system of the amplifier, the design of the amplifier's supporting structures, the amplifier's cooling system, and the thermal, mechanical and optical properties of the amplifier material. Non-uniform heating along with uneven cooling produces temperature gradients within the gain material. As a result, inhomogeneous optical properties develop and this, along with mechanical deformation (i.e., thermal lensing) plus thermal stresses within the medium can further degrade the quality of the beam output. If not properly controlled, thermally induced stress fracture can also occur, which ultimately limits the laser's maximum power output.
Significance of Thermo-Mechanical-Optical Effects
Thermally induced distortion and its effect on optical performance have been studied for rod amplifiers where classical linear theory considers constant thermal, mechanical, and optical properties within the gain material. Much of this work includes yttrium aluminum garnet (YAG) materials that are often used in HAP applications and addresses both steady state and pulsed laser operations. Optical distortions in YAG rod amplifiers caused by the dependence of the material's index of refraction on temperature T and stress σ and have been analyzed extensively. [D. C. Brown, “Nonlinear thermal distortion in YAG rod amplifiers”, IEEE J. Quantum Electron., Vol. 34(12), p. 2383 (1998) and W. Koechner, Solid-State Laser Engineering, 6th Revision, Springer Series in Optical Sciences, New York, N.Y., 2006] FIG. 8 shows the associated stresses experienced by the two thin disk configurations for the temperature profiles of FIG. 3. More recently, measurements of the temperature dependent material properties (Yb:YAG) have enabled much more accurate analysis of the non-quadratic radial and tangential polarizations. [R. Wynne, et al., “Thermal Coefficients of the Expansion and Refractive Index in YAG”, Appl. Opt., Vol. 38, p. 3182 (1999) and D. C. Brown, “Nonlinear thermal distortion in YAG rod amplifiers”, IEEE J. Quantum Electron., Vol. 34(12), p. 2383, (1998)]. Unfortunately, analyses with temperature dependent properties do not yield simple analytical solutions and instead require a numerical approach such as the finite element method. Similarly, the geometry of the thin disk laser does not lend itself to analytical evaluations of distortion, especially since the design of the structure that mounts the thin disk affects its thermo-mechanical-optical response. [Y. Liao, et al., “Pressure tuning of thermal lensing for high-power scaling”, Opt. Lett., Vol. 24(19), p. 1343 (1999)]. Employing the thermo-mechanical properties in the numerical analysis of Yb:YAG yields the radial spatial optical path differences (OPD) experienced per pass. FIG. 9 shows these associated OPD results in Yb:YAG thin disks from the temperature profiles of FIG. 3 being excited at 5 kW/cm2, the deformations of FIG. 5 and the birefringence resulting from the thermal stress of FIG. 8.
Moreover, the cooling technique must be very efficient and at high thermal energy fluxes (>kW/cm2) in order to further increase laser output power. Increase of the pump power to produce higher laser output power naturally results in significantly more heating of the thin disks. The generated heat must be removed as quickly as possible. A good way to attack this issue is to enhance heat transfer from the heat sink by improving the heat sink design and utilizing an advanced thermal management approach. Advanced thermal management that can efficiently and quickly remove heat from TDL and make uniform temperature in a TDL is the most challenging issue for the next generation of high power TDL. The current thermal management technology is limited not only by the heat removal rate but also by the temperature uniformity. Currently, the achieved highest rate of heat removal from a TDL is approximately 0.3 kW/cm2. Scaling the laser output power greater than 10 kW with very good single mode, high BQ thin disk laser operation, this heat removal can become more than 1-2 kW/cm2.
In order to increase the heat removal capability and reach a higher level of temperature uniformity, cooling devices or systems based on phase change heat transfer have been widely utilized. For example, heat pipes are used in laptop or desktop computers. When heat continuously increases in high power TDL's, currently available cooling devices such as liquid cooling employing jet impingement cooling approaches cannot meet the requirement. This is attributed to the facts that the capillary limitation, boiling limitation, vapor flow effect, and thermal resistances occurring in the conventionally configured heat pipe wicks significantly limit the heat transport capability. Therefore, in order to develop a highly efficient cooling system to remove the extra-high heat flux and significantly increase the effective thermal conductivity, a novel mechanically controlled hybrid of advanced high power technology heat pipes is proposed and discussed in detail below.
The value of the higher thermal conductivity from the advanced heat spreader highlighted above described in detail below are shown in FIG. 10 by the comparison of temperature profiles for the two different thermal conductivity, 180 (CuW) and 10,000 W/m*° K. For the latter, the temperature profile through the thin disk is very constant radially except for the very edge of the thin disk. FIGS. 11-12 show respectively the deformation and stress comparisons for these two different thermal conductivities of the effect heat spreaders. Also, note that the maximum temperature at the center of the thin disk is 77° K lower. FIG. 13 shows the corresponding OPD for the temperature profiles of FIG. 10 creating the deformations and stress induced birefringence. The nearly flat OPD clearly shows the real value of the advanced heat spreader to significantly improve the BQ of thin disk lasers. Operating an 80° K should make the BQ very good even at very high power loading. In addition, the de-polarizing losses in the thin disk due to stress-induced birefringence should be greatly decreased and maybe completely eliminated.
Advanced Thermal Management-Micro-Channeled, Closed Loop Oscillating Heat Pipe
For the thermal management aspects of the advanced heat spreader (AHS) to be appreciated, first a discussion of main concepts must be described, namely, thin film evaporation, thermally excited oscillating motion, nanofluid, and nanostructure-modified wicks. The advanced heat spreader system uses capillary heat pipe (HP) action in closed, not open tubes that contain “wicks” to return working flow. FIG. 14 illustrates a conventional “wicked” heat pipe This AHS approach eliminates the reduction of working fluid flow by forcing a unidirectional flow. The large “effective” thermal conductivity is created as a result of the unidirectional flow (often aided by check-valve) and the evaporation—condensation action of working fluid (i.e., the latent heat of vaporization), a thermal process called “oscillating motion of the liquid plugs and vapor bubble”. FIG. 15 shows the basic concept principle of oscillating heat pipes (OHP) which are described below. It should be noted that the “Heat Pipe Action” can be promoted by various “working fluids” such as water, acetone, ammonia and nitrogen (cryogenic operation) at room temperature (RT).
Thin Film Evaporation
In the presence of a thin film, a majority of heat will be transferred through a very small region. [M. A. Hanlon, et al., “Evaporation Heat Transfer in Sintered Porous Media,” ASME Journal of Heat Transfer, Vol. 125, p. 644, (2003); S. Demsky, et al., “Thin film evaporation on a curved surface”, Microscale Thermo-physical Engineering, Vol. 8, p. 285 (2004); H. B. Ma, et al., “Fluid Flow and Heat Transfer in the Evaporating Thin Film Region,” Microfluidics and Nanofluidics, Vol. 4, No. 3, pp. 237-243. (2008).] When evaporation occurs only at the liquid-vapor interface in the thin-film region, in which the resistance to the vapor flow is negligible, evaporating heat transfer can be significantly enhanced, resulting in much higher evaporating heat transfer coefficient than boiling heat transfer coefficient with enhanced surfaces [S. G. Liter, et al., “Pool-boiling CHF Enhancement by Modulated Porous-Layer Coating: Theory and Experiment,” International Journal of Heat and Mass Transfer, 44, pp. 4287-4311 (2001)]. Utilizing this information, a number of high heat flux heat pipes have been developed at the University of Missouri (MU). For example, a micro-grooved heat pipe, 6-mm diameter and 135-mm length produces a temperature drop of only 2° C. from the evaporator to the condenser under a heat input of 50 W. Another example is the air-cooled aluminum heat pipe which can remove a total power of 200 W with a heat flux up to 2 MW/m2. Utilizing and optimizing thin film regions significantly increases the heat transport capability and effectively increase the effective thermal conductibility of the vapor chamber.
Oscillating Motions and Oscillating Heat Pipes
Oscillating single-phase fluid significantly enhances heat and mass transfer in a channel, and has been employed in a number of heat transfer devices. [S. Backhaus, et al., “A Thermo-acoustic Stirling Heat Engine,” Nature, Vol. 399, p. 335 (1999); U. H. Kurzweg, “Enhanced Heat Conduction in Fluids Subjected to Sinusoidal Oscillations,” ASME Journal of Heat Transfer, Vol. 107, p. 459 (1985); U. H. Kurzweg et al., “Heat Transfer by High-Frequency Oscillations: A New Hydrodynamic Technique for Achieving Large Effective Thermal Conductivities,” Physics Fluids, Vol. 27, p. 2624 (1984); M. Kaviany, “Performance of a Heat Exchanger Based on Enhanced Heat Diffusion in Fluids by Oscillation: Analysis,” ASME Journal of Heat Transfer, Vol. 112, p. 49 (1990); M. Kaviany, et al., “Performance of a Heat Exchanger Based on Enhanced Heat Diffusion in Fluids by Oscillation: Experiment,” ASME Journal of Heat Transfer, Vol. 112, p. 56]. The oscillating motions generated by a variable-frequency shaker could result in thermal diffusivity for up to 17,900 times higher than those without oscillations in the capillary tubes; however, the use of mechanically driven shakers may limit its applications to miniature devices. Akachi [U.S. Pat. No. 4,921,041 5,219,020] pioneered a new device, called oscillating heat pipe (OHP), which utilizes the pressure change in volume expansion and contraction during phase change to excite the oscillation motion of the liquid plugs and vapor bubbles. This results in four unique features that do not exist in regular heat pipes: 1) OHP is an “active” cooling device, in that it converts intensive heat from the high-power generating device into kinetic energy of fluids in support of the oscillating motion; 2) liquid flow does not interfere with the vapor flow in high heat removal because both phases flow in the same direction; 3) the thermally-driven oscillating flow inside the capillary tube will effectively produce some “blank” surfaces that significantly enhance evaporating and condensing heat transfer; and 4) the oscillating motion in the capillary tube significantly enhances forced convection in addition to the phase-change heat transfer. Because of these features, the OHP has been extensively investigated in the past several years [H. B. Ma, et al., “Temperature Variation and Heat Transfer in Triangular Grooves with an Evaporating Film,” AIAA Journal of Thermophysics and Heat Transfer, Vol. 11, p. 90 (1997); K. Park, et al., “Nanofluid Effect on the Heat Transport Capability in a Well-Balanced Oscillating Heat Pipe,” AIAA Journal of Thermophysics and Heat Transfer, Vo. 21, No. 2, p. 443 (2007); H. B. Ma, et al., “Heat Transport Capability in an Oscillating Heat Pipe,” ASME Journal of Heat Transfer, Vol. 130, No. 8, p 081501 (2008); S. M. Thompson, et al., “Experimental Investigation of Miniature Three-Dimensional Flat-Plate Oscillating Heat Pipe,” ASME Journal of Heat Transfer, Vol. 131 (4) (2009); Y. Zhang, et al., “Heat Transfer in a Pulsating Heat Pipe with Open End,” International Journal of Heat and Mass Transfer, 45, pp. 755-764 (2002)]. Additionally, there has been cryogenic demonstrations of oscillating heat pipes. [H. Xu, et al., “Investigation on the Heat Transport Capability of a Cryogenic Oscillating Heat Pipe and Its Application in Achieving ultra-Fast Cooling Rate for Cell Verification Cryopreservation,” Cryobiology, Vol. 56, pp. 195-203 (2008)]. The results are so promising that the combination of high-conducting nanofluids with OHP will further the energy carrying capacities. But the current oscillating heat pipe operating only by the thermal excitation cannot produce an extra high level of oscillating frequency. As a result, it is not possible to remove higher thermal heat power flux level more than 0.3 kW/cm2. A mechanically-controlled oscillation motion of two phase flow is shown herein.
High Heat Transport Capability of Nanofluids
High heat transport capability of nanofluids produced by adding only a small amount of nanoparticles into the fluid has qualified nanofluids as a most promising candidate for achieving ultra-high-performance cooling. It has been demonstrated that the dispersion of a tiny amount of nanoparticles in traditional fluids dramatically increases their thermal conductivities. [H. B. Ma, et al., “An Experimental Investigation of Heat Transport Capability in a Nanofluid Oscillating Heat Pipe,” ASME Journal of Heat Transfer, Vol. 128, p. 1213 (2006); H. B. Ma, et al., “Nanofluid Effect on the Heat Transport Capability in an Oscillating Heat Pipe,” Applied Physics Letters, Vol. 88 (14), p. 1161 (2006); H. B. Ma, et al., “An Investigation of Oscillating Motions in a Miniature Pulsating Heat Pipe,” Microfluidics and Nanofluidics, Vol. 2, No. 2, pp. 171-179 (2006); K. Park, et al., “Nanofluid Effect on the Heat Transport Capability in a Well-Balanced Oscillating Heat Pipe,” AIAA Journal of Thermophysics and Heat Transfer, Vo. 21, No. 2, p. 443 (2007)]. Since 1995, outstanding discoveries and seminal achievements have been reported in the emerging field of nanofluids. The key features of nanofluids discovered so far include (a) thermal conductivities far above those of traditional solid/liquid suspensions [J. A. Eastman, et al., “Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol-Based Nano-Fluids Containing Copper Nano-Particles,” Applied Physics Letters, Vol. 78, p. 718 (2001)]; (b) a nonlinear relationship between thermal conductivity and concentration [S. Choi, et al., “Anomalous Thermal Conductivity Enhancement in Nano-tube Suspensions,” Applied Physics Letters, Vol. 79, p. 2252 (2001)]; (c) strongly temperature-dependent thermal conductivity [S. K. Das, et al., “Heat Transfer in Nanofluids—A Review,” Heat Transfer Engineering, Vol. 27(10), p. 3 (2006)]; and (d) significant increase in critical heat flux (CHF) [Y. Xuan, et al., “Investigation on Convective Heat Transfer and Fluid Features of Nanofluids,” Journal of Heat Transfer, 125, pp. 151-155 (2003); I. C. Bang, et al., “Boiling heat transfer performance and phenomena of Al2O3-water nano-fluids from a plain surface in a pool,” International Journal of Heat and Mass Transfer, Vol. 48 (12), p. 2407 (2005)]. These key features make nanofluids strong candidates for the next generation of coolants to improve the design and performance of thermal management systems. Most recently, nanofluids were put into an oscillating heat pipe (OHP) and it was found that nanofluids significantly enhance the heat transport capability in the OHP. [Y. Zhang, et al, “Nano-particle Effect on the Thermal Conductivity of Nanofluids,”: International Journal of Heat and Mass Transfer, Vol. 51 (19-20), p. 4862 (2008)]. As shown in FIG. 16 when the nanofluid (HPLC grade water containing 1.0 vol. % 5-50 nm of diamond nanoparticles) was charged to the OHP, the temperature difference between the evaporator and the condenser can be significantly reduced. For example, when the power input added on the evaporator is 100 W, the temperature difference can be reduced from 42° C. to 25° C. It appears that the nanofluid can significantly increase not only the effective thermal conductivity, but also the convection heat transfer and the thin film evaporation in the OHP. The heat transport capability in the nanofluid OHP depends on the operating temperature. When the operating temperature increases, the heat transport capability significantly increases. And the temperature difference between the evaporator and condenser as shown in FIG. 17 was almost constant as the input power increases, and the investigated OHP with charged nanofluids can reach 0.028° C./W at a power input of 336 W, which might set a record of thermal resistance in the similar cooling devices. FIG. 18 shows the effect of nanoparticles on the thin film evaporation of the working fluid within the OHP.